Differential isoform-specific control of KCC2 function in developing and mature cortical neurons.

The K-Cl cotransporter KCC2 is essential for fast synaptic inhibition in the mature brain. It is encoded by a single gene and expressed as two isoforms: KCC2a and KCC2b, which differ in their N-terminal domains. While KCC2b is predominant, the function of the weakly expressed KCC2a isoform remains unclear. Here, we reveal that KCC2a is a potent, bidirectional regulator of KCC2b membrane stability and function in cortical neurons. In immature neurons, where WNK-SPAK kinase activity is high, KCC2a promotes SPAK-dependent phosphorylation of KCC2b at Thr1007, which likely contributes to hindering its membrane expression and function. Conversely, in mature neurons with low basal WNK-SPAK activity, KCC2a promotes KCC2b expression, clustering, and function. At this stage, although accounting for less than 5% of total KCC2 mRNA, KCC2a is enriched in dendrites and within KCC2 clusters, where it prevents clathrin-mediated KCC2b endocytosis, as well as polyubiquitination and proteasomal degradation. Thus, KCC2a acts as a developmental switch that first inhibits KCC2b during early development and then ensures its membrane stability to support effective synaptic inhibition in the adult brain.